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An anonymous reader writes "The FreeBSD Project has begun the switch of its source code management system from CVS to Subversion. At this point in time, FreeBSD's developers are making changes to the base system in the Subversion repository. We have a replication system in place that exports our work to the legacy CVS tree on a continuous basis.
People who are using our extensive CVS based distribution network (including anoncvs, CVSup, cvsweb, ftp) will not be interrupted by our work-in-progress. We are committed to maintaining the existing CVS based distribution system for at least the support lifetime of all existing 'stable' branches. Security and errata patches will continue to be made available in their usual CVS locations."

They don't use git because FreeBSD development has traditionally been "centralized". They use a model where patches are fed to a group of core developers with "commit permission" to the tree, and all source changes are vetted and fed through that funnel. Subversion's centralized source control methodology works well with the FreeBSD development process, and the decentralized aspects of git is not needed.

However, of course, there is still some distributed coding going on at the edges, but they tend to be peripheral and experimental. The developers working on these experimental branches can choose to use whatever source control system they wish. Many FreeBSD developers prefer perforce for their experimental work, but they can use git or mercurial if they wish.

Emacs is also GPLed, but that doesn't mean everything I write with it is also automatically GPLed. Unless the BSD people are planning to become Git developers, the source license makes no difference. Try reading it sometime.

Path of least resistance; it works much like CVS, it fits in with existing infrastructure, and everyone knows how to use it.

git isn't terribly well suited to very large monolithic projects; you need to split into multiple smaller projects since it tracks entire trees rather than single files. When your tree is 1.3GB+ and has upwards of quarter of a million files that's rather painful either way.

It also isn't well suited to rewriting history, e.g. in the case when you have to remove a changeset because it violates someone's patent or copyright; you can rewrite the repository to remove it, but you end up renaming every commit afterwards, since their names are SHA1's dependent on every previous commit, generating tonnes of churn in many different places as the whole of history basically disappears and reappears elsewhere.

Many of git's advantages can still be leveraged with SVN; git-svn works pretty well, and it doesn't require massive upheavals in all areas of the project.

Mercurial [wikipedia.org] is the solution currently in use at Mozilla, Sun, and quite a few Linux projects. (Though not the main kernel branch. That's GIT. Think something more along the lines of ALSA.)

Mercurial is interesting because it encourages teams to work together, pushing and pulling source code from each other. When the source reaches a stable point, you can push that to a central repository for building and archiving.

The interesting aspect about this design is that it actively encourages branching! Rather than treating branches as a special thing that needs to be done under a certain set of circumstances, it treats every copy of the repository as a branch. So developers can work independently. When they come back together, the tool is able to auto-merge most projects back into a single whole.

Mercurial is able to do this because it tracks the point of divergence. With that information, it can see if any of the changes truly conflict. 95% of the time, there is no conflict and Mercurial is able to merge the files auto-magically. The other 5% of the time, Mercurial will launch a merge tool and make you answer YES/NO to each difference. This process is amazingly smooth.

The key thing to keep in mind with Mercurial is that you won't want to keep all your source in one repository. (Like most companies do with CVS.) Keep a separate repository for each project or module. You can keep the repositories all in the same path, but it's much easier to work with only the code you need rather than copying around a 10GB source tree from developer to developer.

If you do decide to try Mercurial and are given a Windows development machine, I highly recommend TortoiseHG [sourceforge.net]. You'll occasionally have to run 'hg update' from the command line (the tool will prompt you), but it's otherwise a very slick way of working with Mercurial repositories.

Oh, and don't use the CVS->Mercurial conversion tools. It leaves CVS-style droppings all over creation. Just import the latest codebase and keep CVS running in read-only mode for as long as you need historical data.

If you are used to CVS then Subversion is definitely a step up, and it will be very familiar to your users. What's more it is well supported by IDEs and has piles of other tools like Tortoise that makes it easy for non-developers to use. Heck, if push comes to shove it can even be used as a WebDAV share with the advantage that it will automagically version your files.

The downside of Subversion is that it isn't very good at merging. Merging branches in current versions of Subversion is a manual process that is ridiculously painful. This can be mitigated somewhat using the svnmerge Python script, but even with the script merging still isn't as easy as any of the distributed version control systems. For people like Linus Torvalds that's basically a showstopper for Subversion. To them merging is basically the whole point of a version control system.

It's quite possible, however, that you have different needs than the Linux kernel. For example, none of the distributed version control systems deal well with large files. If you want to store multi-media files along with your source then Subversion is basically your only option. Likewise, if you plan on having designers or random office workers use your repository then you can forget about the distributed tools.

It is definitely possible to do branch/merge work with CVS/SVN, but you need to track the merge points yourself, and be aware not to merge the same changeset twice. Git and Mercurial make the process far easier by tracking merge points for you.

Also, one big thing distributed source control gives you is the ability to "clone" or "fork" your own copy of the repository. This lets you make small, incremental changes that you can roll-back and merge back as you like, and then you can "push" all of your commits back to another fork (presumably a dev trunk somewhere). As an example, check out this visual representation of the many forks of the Rails project on Github: http://github.com/rails/rails/network [github.com]

Perforce is a central repository for internal development. That way the limitations of CVS for this part of the job don't limit the developers.But Perforce is commercial software and you can't push it on to the community.

Subversion is a free software which has the capabilities which are set as a requirement for the FreeBSD project. It has some capabilities of Perforce, it has some capabilities of CVS and it can be integrated in the current distribution framework.

Yes, try the same search with quotes. Not that it would matter anyways, as it's not a viable method for comparing the usage of both. Let me reinterpret these results:

"Git is easier to learn than Mercurial as evidenced by the sheer ammounts of Mercurial tutorials that seem to be needed"

(Disclaimer: This is not my personal opinion. Just to demonstrate the uselessness of this data. But flame away anyways if you have bad eyesight, I'll actually be doing something worthwhile in the meantime.)

Mercurial [wikipedia.org] is the solution currently in use at Mozilla, Sun, and quite a few Linux projects. (Though not the main kernel branch. That's GIT. Think something more along the lines of ALSA.)

ALSA does not use Mercurial any more, they switched to GIT on 2008-05-20.

Windows or Unix shop? How large do you expect your repository's history to get (in terms of revisions per year)?

For a UNIX shop with a very deep history, you're going to be best off with Git.

For a UNIX shop without an exceptionally deep history, Bazaar (Canonical's revision control system) is worth considering. It works well on Windows, but the GUIs there are immature -- so if you're not a UNIX shop and your Windows folks are revision control power users who hate the command line (for whom the currently available GUIs aren't adequate but the command line is unsuitable), that's something which will need to be taken into consideration.

For a Windows shop where you need the best available GUIs, Subversion has the stability you need right now -- but you're missing out on functionality which can save a lot of headaches later (when, say, you try to merge between a branch where you did a mass renaming and one where you didn't... though Git doesn't always handle this one well either, Bazaar takes care of it beautifully). SVN 1.5 will be slightly better along those lines, and bzr-svn (allowing Bazaar to integrate with Subversion repositories) can help your UNIX power users take advantage of distributed revision control's advantages while still keeping all the SVN shininess available for your Windows folks.

That said -- SVN set out to be the best replacement for CVS it could possibly be. It's a very nice, very polished replacement for CVS -- but that's not the same thing as "the best revision control system it could possibly be"; distributed workflows have some nontrivial advantages even in centralized environments; look at the PQM the Bazaar folks use to manage their own project for a prime example.